Electric valve control system



Sept. 23, 1941. H. WINOGRAD ELECTRIC VALVE CONTROL SYSTE? Filed June 28, 1940 Patented Sept. 23, 1941 UNITED STATES. PATENT OFFICE 2,256,755 morale VALVE comaor. srs'mm Harold Winograd, Milwaukee, Wis, asaignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application June 28, 1940, Serial No. 342,851

17 Claims. I (Cl. 171-97) This invention relates in general to improveconverter in the examp e co idered a ove- The ments in electric valve control systems and more induc ance of th l d circuit y 081158 this particularly to means for preventing overloading current t continue t0 flow during a P d e of an electric current converter of the electric ins easily ten times the length of a normal pevalve type as a result of changes in the operatriod of current now through a valve, thereby ing condition of another converter connected in overloading the valve or group of valves to the parallel therewith. point of injury or even of possible destruction.

The supply of direct current of high inten- Interruption of the flow of current through sity for electro-chemical processes is now genthe converters is therefore preferably effected by erally effected through electric current convert.- simultaneously opening the circuit breakers sevins systems of the electric valve type. The erally controlling the flow of current through amount of energy involved then frequently bethe different conver rs n O y thereafter ncomes so large that the current required for a dering the valves of the converters non-conducplant cannot be supplied by a single converter, tive to permit subsequent reclosure of the circuit so that a plurality of converters are connected in b ea ers. parallel between the supply circuit and the load Several load circuits of the type above concircuit. For example, metallic aluminum is now sidered may also be connected in parallel to obtained in series connected electrolytic cells reobtain a uniform distribution of the flow of curquiring current reaching 50,000amperes, which rent through the converters connected with all may be obtained from a battery of ten parallel the load circuits, even when the load circuits converters each capable of delivering current of are carrying currents of different values or when 5,000 amperes. It is then impractical to control one of the converters is temporarily disconnected the current flowing through the cells by means from the associated load circuit. The control of a single circuit breaker connected in series circuits for the diiferent groups of converters therewith because the size and weight of the 2 are then preferably interlocked with the switching moving parts of the circuit breaker reach such means interconnectins the load circuits to cause magnitudes as to render the operation thereof disconnection of any load circuit when the conunreliable. verters associated therewith are rendered inop- It is therefore preferable to control the indierative, and to render inoperative only the convidual converter currents either by means of converters of one of the load circuits upon urtrol electrodes of the valves or of a plurality of rence of a permanent disturbance in one of such circuit breakers each controlling the flow of curconverters. rent through one of the converters. Initiation It is therefore one object of the present inof the flow of current simultaneously through the vention to p o d a nt ol syst m for a p udiiferent converters is preferably efiected by comrality of parallel current converters oi the elecmou means rendering the valves of the diiferent tric valve type for insuring simultaneous initiaconverters conductive simultaneously. tion of the now of current through the convert- If the valves of the different converters opercm to a common load circuit. ate during diil'ercnt periods, as is generally the Another object of the present invention is to case in practice, it is undesirable to interrupt the 40 provide a control system for a plurality of parflow of current through the converters by reriderallel current converters of the electric valve type ing the valves non-conductive. The reason for preventing overloading of one of the contherefor is that if it is attempted to render all verters while another of the convertersis being the valves of a converter non-conductive the disconnected from a common load circuit.

last valve to carry current continues to remain Another object of the present invention is to in operation until the voltage impressed between provide a control system for a plurality of paralthe anode and cathode thereof is reversed. This lel current converters of the electric valve type voltage reversal takes place sequentially in the. for preventing overloading of one of the condiiferent converters so that the converters beverters when the load circuits associated with come inoperative in sequence. When all the different groups of converters are connected in converters except one have become inoperative, parallel.

one valve or one group of parallel valves of the Another object of the present inventionlis to last converter to operate momentarily carries the provide a control system for a plurality of parentire load current of the system, which may allel current converters oi. the electric valve type begten times the maximum normal current of one for causing all the converters to become inopcondition takes place in one of the converters.

Objects and advantages other than those above described will be apparent from the following description, when read in connection with the accompanying drawing, which diagrammatically illustrates one embodiment of the present invention applied to the control of two groups of converters severally associated with two parallel load circuits.

Referring more particularly to the drawing by characters of reference, reference numeral 6 designates a polyphase alternating current circuit to be connected with a direct current circuit I through a plurality of parallel converters 8, 9 of the electric valve type. The flow of energy through the converters may take place in either direction, but it will be assumed that current is beingtransmitted therethrough from circuit 6 to circuit I. Circuit 6 is therefore a supply circuit k connected with a suitable current generator (not shown) and circuit 1 is a load circuit connected with current consuming devices of high capacity,

such as electrolytic cells In. Although the ad-v vantages of the present invention increase with the number of converters connected in parallel, only two converters are shown connected with circuit I to simplify the drawing and the description thereof.

Converter 8 comprises a main transformer H and a plurality of electric valves generally designated by i2. Transformer H comprises a pri- "mary winding 13 and a secondary winding l4.

Winding I3 is connected with circuit 6 through a switch It provided with a trip coil l6a connected with a current transformer I! inserted in one of the connections of winding l3 with circuit 6. Winding i4 is connected in star and is preferably divided into two groups of phase portions defining two neutral points severally connected with the negative conductor of circuit 1 through an interphase transformer I83.

Valves l2 are severally provided with anodes l9 which are preferably arranged within a com mon casing, the cathodes of the valves then being combined into a common cathode structure 7 20. Suitable means (not shown) are provided for bringing cathode into electron'emitting condition and for maintaining the cathode in such condition, as is well known. Anodes l9 are severally connected with the phase portions of winding l4, and cathode 20 is connected with the positive conductor of circuit 1 through a switch 2] provided for controlling the flow of current through converter 8. Switches l6, 2! may also be replaced by a single multipole switch inserted in the connections between winding l4 and anodes H as is well known. The connections of winding l4 and cathode 20 with cells l0, including a portion of circuit 1, are usually of such length as to present a material amount of inductance which may be assumed to be localized in an inductive reactor 22.

The conductivity of valves I2 is controlled by 'means of control electrodes 23 severally associated with anodes l9 and which may ke variably energized to vary the operation of converter 8. Each control electrode 23 is connected with cathode 20 through a circuit comprising a current limiting resistor 24 and a source of negative potential such as a battery 26 serving to maintain the valves non-conductive. To render the converter operative, valves 12 are rendered conductive by means of a transformer 21 having a sec- .ondary winding divided into phase portions sev- -erative when a permanent abnormal operating erally inserted into the control electrode circuits. The primary winding of transformer 21 is energized from circuit 6 through contacts 28a, 28b of a relay 28.

Switch 2| may be closed in any desired manner and is held closed against the action of an opening spring 2|a by means of a latch 2"). The switch is arranged to open upon occurrence of an operating condition of converter 8, such as a reverse flow of current through cathode 20, by

means of a series trip coil 2Ic acting on a polarized armature 2ld adapted to withdraw latch -2lb. Thisarrangement is designed to respond within the shortest possible time. The latch may also be withdrawn by energizing a shunt trip coil 2| e.

Coil 2Ie may be connected with a battery 29 through. contacts of. a reverse current relay 30 comprising a coil 30a serially connected with cathode 20 and acting on a polarized armature- 30b. Relay 3!) is designed to respond to the flow of reverse current'through coil 301: with a very short time delay to cause the relay to operate only if switch 2! fails to open.

Converter 9 is similar to converter 8 and comprises a second plurality of electric valves,3l provided with anodes 32 and a cathode 33. Anodes 32 are severally connected with the phase portions of secondary winding 34 of a second main transformer 36 energized from circuit 6 through a switch 31 similar to switch I6. Winding 34 is preferably similar to winding l4 and is connected with circuit 1 through. an.interphase transformer 38. The primary windings i3, 39 of transformers H and 3B are,- however, different from each other to cause the terminal voltages of winding 34 tobe displaced in phase with respect to the terminal voltages of winding l4, whereby the voltages of windings l4 and 34 jointly form a polyphase system of voltages.

The conductivity of valves 3| is controlled by means of control electrodes 40 each connected with cathode 33 through a battery 4|, one of the secondary winding phase portions of a transformer 42 and a current limiting resistor 43. Transformers 21 and 42 are so arranged that the secondary voltages thereof are in the same phase relation as the voltages of windings I4 and 34 to cause valves 3! to operate in the same manner as valves l2. Cathode 33 is connected with circuit I through a second switch 44 for controlling the flow of current through converter 9. Switch 44 is provided with a polarized tripping mechanism similar to that of switch 2| and with a shunt trip coil 44c connected in parallelwith trip coil Zle. A second polarized relay 46 responsive to reverse flow of current through cathode 33 is provided with contacts connected in parallel with the contacts of relay 30.

Circuit 6 may be connected with a second load circuit 41 through a phase shifter 48 and through a plurality of parallel converters 49, 50. Circuit 31 is connected with. load devices such as electrolytic cells. 5| similar to cells In. Converter 49 is identical to converter 9 and likewise comprises a plurality of electric valves 52 and a transformer 53 energized through a switch 55 and having a secondary winding 54. The flow of current through converter 49 may be interrupted by means of a switch 56 provided with a trip coil 56e adapted to be connected with battery 29 through the contacts 01 a polarized relay 5']. The conductivity of valves 52 may be controlled 'by means comprising a transformer 58 energized from circuit 6 through phase shifter 48 and through the contacts 59a, 59b of a relay 59'.

Converter 50 is identical to converter 8 and likewise comprises a plurality of electric valves 68 and a transformer 6| energized through a switch 65 and having a secondary winding 62. The flow of current through converter 50 may be controlled by means of a switch 63 provided with a trip coil 63e connected in parallel with trip coil56e. The connection between switch 63 and circuit 41 comprises a polarized relay 64 provided with contacts connected in parallel with the contacts of relay 51. Th conductivity of valves 60 may be controlled by means comprising a transformer 66 connected with phase shifter 48 through contacts 59a, 59b of relay 59. The voltages of windings 54 and 62 form a system of polyphase voltages similar to the system of voltages of windings I4, 34. By reason of the connection of transformers 53, 6| with circuit 6 through phase shifter 48, however, the

ode potential by means of batteries 26, 4|. As

a result thereof the valves are rendered nonvoltages of windings 54, 62 are displaced in phase with respect to the voltages of windings I4, 34 and the voltages of the four windings jointly form another polyphase system of voltages.

Circuit 41 may be connected in parallel with circuit 1 by means of switches 61, 68. Switch 68 is provided with a plurality of auxiliary contacts 68a to 660. The closing coil 68d of switch 68 may be connected with battery 29 through contacts 280, 68a and 590 and through a switch 69. Switch 68 is accordingly interlocked with switches 28, 5,9 and is caused to close in response to the operation thereof. Contacts 68b control the connections of battery 29 with coils M6, 44c

through contacts 28d, and with coils 56c, 63c

through contacts 59d.

Switches 2|, 44 and switches 56, 63 may selectively be caused to open by means of a pair of control switches 10, 1| and of contacts 680 of switch 68. Switch 10 is provided with a plurality of contacts movabl into two control positions and tending to return to the rest position shown. One of the contacts 180. of switch 18 controls the connection of trip coil 68c with battery 29. Trip coil 68c may also be energized from battery 29 through a switch 12, through contacts 28c, contacts 59c, or contacts of relays 38, 46, 51

and 64. Another contact 181) controls the connection of coils 2| e, 44 with battery 29 through contacts 680. Switch 18 is further provided with contacts 10c, 18d for connecting coil 28} with battery 29. Switch 1| is identical to switch 18 and is connected similarly theretofor controlling the operation of the switches 56, 63 and of relay 59.

In operation, the system being connected as shown on the drawing and circuit 6 being energized, the supply of current to circuit 1 may be initiated by a sequence. of operations commencing with rendering cathodes 28 and 33 emissive in a known manner. The load represented by cells l8 may be assumed to require the full output of converters 8, 9 and of any other converters connected in parallel therewith, so that if a converter were operatively connected alone with circuit 1, the converter would be severely overloaded. The flow of current should therefore be initiated simultaneously through all the parallel converters connecting circuit 6 with circuit 1.

For this purpose, switches 2|, 44, I6 and 31 are first closed to complete the connections of the converters with circuits 6 and 1. The control electrodes of valves l2, 3| are then maintained conductive for the flow of current therethrough. Switch 16 may thereafter momentarily be moved to the left to close contact 18d. Contact TM is a so-called slip contact which remains closed when switch 10 is returned to the position shown.

Closure of contact 10d connects coil 28; with battery 29 and relay 28 closes contacts 28a, 28b, 28c and opens contacts 28d, 28c. Transformers 21 and 42 are thereby simultaneously connected with circuit 6 and therefore simultaneously become operative. Transformer 21 immediately impresses a positive potential on at least one control electrode 23 associated with an anode |9 receiving a positive potential from winding |4, thereby releasing the flow of current through such anode. At the same time, transformer 42 impresses a positive potential on at least one control electrode 40 associated with an anode 32 receiving a positive potential from Winding 34 to release the flow of current therethrough. Operation of relay 28 thus immediately causes a valve of ach converter to be rendered conductive so that the flow of current is initiated simultaneously through the two converters and neither converter becomes overloaded.

After the initiation of the flow of current through the converters, the different valves 22 operate in sequenceand carry current during predetermined periods depending on the phase relation of transformers II and 21 and on the voltage of battery 26, as is well known. Valves 3| operate during predetermined periods different from the operating periods of valves |2 depending on the phase relation of transformers 36 and 42 and on the voltage of battery 4|. Because of non-coincidence of the operating periods of valves l2 and 3|, it is not desirable to interrupt the flow of current between circuits 6 and 1 by returning relay 28 to the position shown. This procedhre would cause the total current of cells In to continue to flow through either a single valve l2 or a single valve 3| during a length of time depending on the inductance of circuit 1.

To interrupt the flow of current simultaneously through converters 8 and 9, switch 18 is moved toward the right. During thi movement of the switch, contact 100 first closes to maintain coil 28f energized, and contact 16d is thereafter returned from the closed position previously reached to the position shown. Finally, contact 16b closes a circuit from battery 29 through contacts 68c, contact 181) and coils 2 le, Me in parallel back to the battery. Switches 2| and 44 open simultaneously, thereby preventing any overloading of either converter until complete interruption of the flow of current between circuit 6 and 1. Switch 10 may then be allowed to return to the position shown. Contact 180 opens and, as contact 16d was already in the position shown, coil 28f is deenergized. Relay 28 returns to the position shown to disconnect transformers 21 and 42 from circuit 6 and thereby render valves l2, 3| non-conductive.

While converters 8, 9 are connected with circuit 1 to supply current thereto, if it is desired at a negative potential with respect to the cath- 41, the cathodes of valves 52, 68 are rendered emissive, switches 56, 63, 55 and 65 are closed and switch 69 may also be closed. Switch 1| i then moved toward the right, whereby relay 59 is caused to operate to render converters 49, 50 operative in the manner above described with respect to converters 8, 9. Upon operation of relay 59, a circuit is completed from battery 29 through contacts Ha, 28c, switch 69, contacts 68a, closing coil 68d, and contacts 590 back to the battery. Switch 68 is closed by the closing coil and is maintained closed by a latch (not shown) provided therefor. Converters 8, 9, 49, 5!] then operate in parallel to supply current jointly to circuits l, ll. As a result of the connection of transformers 53, M with circuit 6 through phase shifter d8, valves 52, 60 operate during predetermined periods different from the operating periods of valves l2, 3!.

During parallel operation of all converters, if a disturbance such as a backfire occurs in one of valves 12, for example, converters 9, 6,9 and 50 Q9, 50 may likewise selectively be rendered incause a direct current to flow through the series coil of relay 30, switch 2|, coil 21c and cathode 2|] to the backfiring anode 19 in the direction opposite to the normal direction of flow. The fiow of this current through coil 2|c causes armature Bid to withdraw latch Zlb with the result that switch 2| is opened by spring 2m. Switch 2| interrupts the flow of current through cathode 26, thereby precluding operation of relay 30. At the same time an abnormally high current is supplied from circuit 6 to transformer'll as a result of the backfire. This current, flowing through current transformer I1, causes suificient current to be supplied to the trip coil of switch Hi to trip the' switch, which disconnects transformer l I from circuit 6. Converters 9, 49 and 50 remain in operation and jointly continue to supply current 'to circuits 1, d1.

If switch 2| fails to open in response to the reverse flow of current through coil Zlc, relay 30 closes the contacts thereof after a short time delay in response to the same reverse flow of current. Relay 30 connects coils Zle, Me and 686 with battery 29, thereby causing switches M and B8 to open even if switch 2| still fails to open. Opening of switches M, 68 completely interrupts the flow of reverse current to cathode 20 as well as the normal flow of current to cells H], and converters d9, 50 continue to supply current to circuit 41 in the normal manner thereof.

When switch 68 is closed and all the converters are in operation, failure of the coil of relay'28 or of relay 59 results in the converters controlled'by the faulty relay being rendered non-conductive.

Switch 68 should then be opened to prevent overloading of the converters remaining operative. If coil 28) fails, for example, relay 28 returns to the position shown. Switch 68 then being assumed closed, coils 2| e, 44c are connected with battery 29 through contacts 28d and 68b. Coil title is connected with the battery through contacts 286. Switches 2|, 44 and 68 open simultaneously, thereby leaving converters 49, 50 connected with circuit 41 to the exclusion of circuit 1.

During parallel operation of all converters, switches 2|, 44 may be selectively opened independently of switches 56, 63 by movement of switch ill to the right. Contact 10a then closes to connect trip coil 68c with battery 29. The trip coil causes the toggle of switch 68 to collapse and the switch opens. Contacts 680 are closed in response to opening of switch 68 to cause opening of switches 2|, 44 in the manner above described. Switch Ill may then be returned to the position shown to open contact 100 and thereby render valves I2, 31 inoperative. Converters 49, 50 then remain alone in operation to supply current from circuit 6 to circuit 41. If it is desired instead to maintain converters l, 9 in operation, converters operative by means of switch H in the manner above described with respect to converters 8, 9.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In an electric translating system, a supply circuit, a load circuit, a current converter for connecting said supply circuit with said load circuit, means for rendering said current converter operative, a second load circuit, said load circuits being operable independently of each other, a second current converter for connecting said supply circuit with said second load circuit, means for rendering said second current converter operative, a switch for connecting said load circuits in parallel, and means interlocking said switch with the first and second said means.

2. In an electric translating system, a supply circuitfa load circuit, a current converter for connecting said supply circuit with said load circuit, means for rendering said current converter operative, a second load circuit, said load circuits being operable independently of each other, a second current converter for connecting said supply circuit with said second load circuit, means, for rendering said second current converter operative, a switch for connecting said load circuits in parallel, and means responsive to operation of the first and second said means for closing said switch.

3. In an electric translating system, a supply circuit, a load circuit, a current converter comprising an electric valve for connecting .said

supply circuit with said load circuit, means for rendering said valve conductive, a second load circuit, said load circuits being operableindependently of each other, a second current converter comprising a second'electric valve for connecting said supply circuit with said second load circuit, means for rendering said second valve conductive, a switch for connecting said load circuits in parallel, and means responsive to operation of the first and second said means for closing said switch.

4. In an electric translating system, a supply circuit, a load circuit, a current converter for connecting said supply circuit with said load circuit, a first switch for controlling the flow of current through said converter, 3. second load circuit, said load circuits being operable independently of each other, a second current converter for connecting said supply circuit with said secondload circuit, a second switch for connecting said load circuits in parallel, and means for causing opening of said first switch comprising means for causing opening of said second switch and means responsive to opening of said second switch.

5. In an electric translating system, a supply circuit, a load circuit, a current converter for connecting said supply circuit with said load circuit, a first switch for controlling the flow of current through said converter, a second load circuit, said load circuits being operable independently of each other, a second current converter for connecting said supply circuit with said second load circuit, a second switch for controlling the flow of current through said second converter, a third switch for connecting said load circuits in parallel, and means for selectively causing opening of said first and second switches comprising means for causing opening of said third switch and means responsive to opening of said third switch.

6. In an electric translating system, a supply circuit, a load circuit, a current converter comprising an electric valve for connecting said supply circuit with said load circuit, a first switch for controlling the fiow of current through said converter, means for rendering said valve conductive, a second load circuit, a second current converter for connecting said supply circuit with said second load circuit, a second switch for connecting said load circuits in parallel, means for rendering the first said current converter inoperative comprising a control switch movable into a control position and tending to return to a rest position, means responsive to movement of said control switch into said control position for causing opening of said second switch, means responsive to opening of said second switch for causing opening of said first switch, and means responsive to return of said control switch into said rest position for rendering the first said means inoperative.

'7. In an electric translating system, an alternating current circuit, a direct current circuit, a first current converter for connecting said alternating current circuit with said direct current circuit comprising a plurality of electric valves operating during predetermined periods, a first switch for controlling the fiow of current through said converter, means for rendering said valves conductive, a second current converter connected in parallel with said first converter and comprising a second plurality of electric valves operating during periods difierent from the first said periods, a second switch for controlling the fiow of current through said second converter, means for rendering the second said valves conductive, and means for interrupting the fiow of current between said circuits comprising means for causing said first and second switches to open simultaneously and for thereafter rendering the first and second said means inoperative.

8. In an electric translating system, an alternating current circuit, a direct current circuit, a first current converter for connecting said alternating current circuit with said direct current circuit comprising a plurality of electric valves operating during predetermined periods, a first switch for controlling the fiow of current through said converter, means for rendering said valves conductive, a second current converter connected in parallel with said first converter and comprising a second plurality of electric valves operating during periods difierent from the first said periods, a second switch for controlling the fiow of current through said second converter, means for rendering the second said valves conductive, and means for initiatmg the operation of said converters comprising means for causing closure of the first and second said switches and for thereafter causing the first and second said means to become operative simultaneously.

9. In an electric translating system, a supply circuit, a load circuit, a current converter for connecting said supply circuit with said load circuit, a first switch for controlling the fiow of current through said converter, a second load circuit, a second current converter for connecting said supply circuit with said second load circuit, a second switch for connecting said lead 10. In an electric translating system, an alternating current circuit, a direct current circuit, a first current converter for connecting said alternating current circuit with said direct current circuit comprising a. plurality of electric valves operating during predetermined periods, a first switch for controlling the fiow of current through said converter, a second current converter connected in parallel with said first converter and comprising a second plurality of electric valves operating during periods difierent from the first said periods, a second switch for controlling the fiow of current through said second converter, and means for interrupting the fiow of current between said circuits comprising means for causing simultaneous opening of said first and second switches.

11. In an electric translating system, an alternating current circuit, a direct current circuit, a first current converter for connecting said alternating current circuit with said direct current circuit comprising a plurality of electric'valves operating during predetermined periods, means for rendering said valves conductive,'a second current converter connected in parallel with said first converter and comprising a second plurality of electric valves oeprating during periods diflerent from the first said periods, means for rendering the second said valves conductive, and means for initiating the operation of said converters comprising 'means for causing the first and second said means to become operative simultaneously.

12. The method of interrupting the fiow of current simultaneously through a plurality of parallel electric current converters of the electric valve type comprising the steps of simultaneously opening the circuits of said converters and of thereafter rendering the valves of the diflerent converters non-conductive.

13. The method of initiating the flow of current simultaneously through a plurality of parallel electric current converters of the electric valve type comprising the steps or closing the circuits of said converters and of thereafter rendering a valve of each converter simultaneously conductive.

14. In an electric translating system, a supply circuit, a load circuit, a first current converter for connecting said supply circuit with said load circult, a first switch for controlling the flow of current through said converter, a second current converter for connecting said supply circuit with said load circuit, a second switch for controlling the flow of current through said second converter, a second load circuit, a third converter for connecting said supply circuit with said second load circuit, a third switch for connecting said load circuits in parallel, means responsive to an operating condition of said first converter for causing opening of said first switch, and time delay means responsive to said operating condition for causing opening of said second and third switches.

15. In an electric translating system, a supply circuit, a loadcircu-it, a first current converter 1.01 connecting said supply circuit with said load circuit, a first switch for controlling the flow of current through said converter, a second load sponsive to an operating condition of said first.

converter i'orcausing simultaneous opening of said first and second switches.

16. In an electric translating system, a supply circuit, a load circuit, a first current converter for connecting said supply circuit with said load circuit, a first switch for controlling the flow of current through said converter, a second load circuit, a second current converter for connecting said supply circuit with said second load circuit, a second switch for conneeting said load circuits in parallel, and means for causing opening of said first switch in response to an operating condition of said first converter when said second switch is closed comprising an element of said second switch.

17. The method of controlling the flow of current through a plurality of parallel electric current converters oi the electric valve type comprising the steps of simultaneously opening the circuits of said converters, rendering the valves of the different converters non-conductive, reclosing the circuits of said converters while said valves are maintained non-conductive, and rendering a valve of each converter simultaneously conductive.

HAROLD WINOGRAD.

DISCLAIMER 2,256,755..Harold.Winog1-ad, Milwaukee, Wis. ELECTRIC VALVE CONTROL Sxsnm. Patent dated September 23, 1941. Disclaimer filed February 19, .1943, by the assignee, Allis-Chalmers Manufacturing Company. Hereby enters this disclaimer to claims 8, 11, 13, and '17 in said specification.

[Official Gazette March 16', 1943:]

DISCLAIMER 2,256,755..Harold.Winog1-ad, Milwaukee, Wis. ELECTRIC VALVE CONTROL Sxsnm. Patent dated September 23, 1941. Disclaimer filed February 19, .1943, by the assignee, Allis-Chalmers Manufacturing Company. Hereby enters this disclaimer to claims 8, 11, 13, and '17 in said specification.

[Official Gazette March 16', 1943:]

D I S C LA I M E R 2,256,755.-..Harold.W'inograd, Milwaukee, Wis. ELECTRIC VALVE Common Sysmu. Patent dated September 23, 1941. Disclaimer filed February 19, .1943, by the assignee, Allis-Chalmers Manufactufing Company. Hereby enters this disclaimer to claims 8, 11, 13, a (117 in said specification.

[Ofiicial Gazette March 16, 1948:] 

